Fluid impeller

ABSTRACT

A fluid impeller, such as a fan for moving air or gases, includes one or more rotatable fluid impelling blades; a flow guiding member adjacent to an edge of the blades, and at least one blade facing surface of the flow guiding member being formed substantially as a segment of a sphere of radius r 1  about a center of curvature. Each blade has a flow guiding member abutting edge curved substantially to fit against a sphere of radius r 1 .

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from PCT/GB96/01965, filed Aug. 9, 1996which in turn claims priority from GB 9516398 filed Aug. 10, 1995.

FIELD OF THE INVENTION

The present invention relates to fluid impellers used with mixed flowfans such as fans for moving air or other gases.

BACKGROUND OF THE INVENTION

Various motor-driven fan configurations have been proposed to meetrespective different requirements for performance, noise generation, andcost, etc. A better understanding of these motor-driven fanconfigurations may be obtained from the following description whichreferences FIGS. 1 through 7.

FIGS. 1 and 2 show schematic end and cut-away side views respectively ofan axial flow fan blade assembly. The impeller includes a number ofaerofoil blades 10 fixedly mounted to a central hub 20, which isgenerally enclosed within a stationary cowling 30. The impeller isdriven by a motor 40, and air is driven by the impeller in a direction50 which is substantially along the axis of rotation of the fan bladeassembly.

Axial flow fans provide large volume flow rates of air, but operate atrelatively low pressures. As the pressure increases, the fan is liableto stall.

FIGS. 3 and 4 of the accompanying drawings show schematic end andcut-away side views of a prior art centrifugal fan. This type of fancomprises blades 60 fixedly mounted to a rotating hub 70 driven by amotor 80. The centrifugal fan has a casing 90 which allows air to entergenerally along the axis of rotation of the blade assembly but to exitperpendicular to the entry direction.

In the centrifugal fan, air is forced to rotate by movement of theblades 60 and is flung outwards towards the exit port 100 by thecentrifugal effect. Centrifugal fans are recognized for their low volumeflow rates of air but high pressure performance, generally without thestalling problems exhibited by axial flow fans. However, centrifugalfans are generally not suitable for use with large volume flow rates.

The so-called mix flow fan was developed as a compromise between theaxial and centrifugal fan assemblies. It is designed to operate atgenerally higher pressures than an axial flow fan, but to provide agenerally greater volume flow rate than a centrifugal fan. FIGS. 5 and 6are schematic and cut-away side views respectively of a prior art mixedflow fan.

The mixed flow fan comprises a number of blades 110 attached to acentral frusto-conical hub 120 and to a generally frusto-conical shroud130. The blades 110, hub 120, and frusto-conical shroud 130 form acomplete rotating assembly, driven by a motor 140.

In operation, the fan behaves as a combination of the axial andcentrifugal flow devices, so that air entering the shroud 130 is drawninto the impeller, with a velocity component along the axis of rotation,but the air is also driven outwardly in a similar manner to thecentrifugal fan, with a velocity component perpendicular to the axis ofrotation. These two velocity components combine to give an outputdirection 150 as illustrated in FIG. 6.

FIG. 7 is a typical pressure-volume flow rate performance graphcomparing the performance of the prior art fans shown in FIGS. 1 through6.

In FIG. 7, the first curve 160 illustrates the high-pressure, low volumeperformance of a centrifugal flow fan. The second curve 170 representsthe high-volume, low-pressure operation of an axial flow fan. (Thestalling characteristic of the axial flow fan is not shown on FIG. 7.)The third curve 180 shows the performance of a mixed flow fan whichprovides a generally higher volume but lower pressure performance incomparison to the centrifugal fan, and a higher pressure but lowervolume performance in comparison to the axial flow fan.

Each of the performance curves shown schematically in FIG. 7 relates toa particular prior art fan configuration (fan diameter, number of bladesand angle of blades) and rotation speed of the blade assembly. Oncethese fan characteristics have been set, the fan performance isgenerally fixed, so that, for example, if the operating pressure for thefan is specified, the resulting volume flow-rate which will be obtainedis defined by the performance curve.

However, it is desirable in manufacturing and installing fans to be ableto vary the performance of the fans. This allows a manufacturer tomarket a range of fans having different performance curves, but whichshare some or all of their components in common.

In the case of an axial flow fan, it is relatively easy to vary thefan's performance while still using the same mechanical components. Forexample, the blade angle of incidence can be varied to give dramaticchanges in the performance characteristics. In one example, a change inthe blade angle of incidence from, say, 10° to 40° could result in 2:1change in volume flow rate (and a correspondingly large change indriving power consumption).

However, in the centrifugal and mixed flow fans described above, thereis little room for changing the fan's performance. The number of bladescan be varied, but this tends to give dramatic, rather than gradual,changes in performance. The motor speed can be varied, but this requireseither a belt drive system, which adds to the mechanical complexity ofthe fan, or the use of different motors, such as two-pole, four-pole,six-pole motors, etc. However, since the rotation speed of a two-polemotor is twice that of a four-pole motor, this again leads to dramatic,rather than gradual, variations in the fan's performance.

In summary, none of the previously proposed fans described above providerelatively high pressure operation and still allow the fan performanceto be easily varied.

SUMMARY OF THE INVENTION

This invention provides a mixed flow fan which provides relatively highpressure operation while still allowing the fan performance to be easilyvaried. Specifically, the fluid impeller of the fan of the presentinvention includes one or more rotatable fluid impelling blades and ahub adjacent to an edge of the blades. At least one blade facing surfaceon the hub is formed substantially as a segment of a sphere of radius r₁about a center of curvature. Surrounding the blades is a shroud which isadjacent to an outer edge of each blade. At least one blade facingsurface of the shroud is formed substantially as a segment of a sphereof radius r₂ about the center the curvature. Each blade has a hubabutting edge curve which substantially fits against a sphere of radiusr₁ and a shroud abutting edge curved substantially to fit against asphere of radius r₂ so that each blade is attachable to the hub and tothe shroud at various angles about an axis passing through the center ofcurvature while the hub abutting and shroud abutting edges of the bladeremain substantially abutting the hub and the shroud respectively.

BRIEF DESCRIPTION OF THE FIGURES

The embodiments of the invention will now be described, by way ofexample only, with reference to the accompanying drawings throughoutwhich like parts are referred to by like reference numbers, and inwhich:

FIG. 1 is a schematic end view of a prior art axial flow fan;

FIG. 2 is a schematic cutaway side view of the fan of FIG. 1;

FIG. 3 is a schematic end view of a prior art centrifugal fan;

FIG. 4 is a schematic cutaway side view of the fan of FIG. 3;

FIG. 5 is a schematic end view of a prior art mixed flow fan;

FIG. 6 is a schematic cutaway side view of the fan of FIG. 5;

FIG. 7 is a schematic pressure-volume performance graph for the priorart fans shown in FIGS. 1 through 6;

FIG. 8 is a schematic side view of a fan according to an embodiment ofthe invention; and,

FIG. 9 is a schematic sectional side view of a fan blade for the fan ofFIG. 8.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring now to FIG. 8, a fan according to the preferred embodiment ofthe invention comprises a number of blades 200 held between a rotatinghub 210 and a rotating shroud 220, so that the hub 210, blades 200 andshroud 220 form a single rotating assembly 190. The single rotatingassembly 190 is driven by a motor 230, coupled to the hub 210 via abracket 240.

The fan operates similar to the mixed flow fan described above, so thatair enters in a generally axial direction 250 at an entrance of therotating shroud 220, and is driven axially and outwardly by the rotatingblades 200 to emerge in an exit direction 260.

The blade angle can be easily adjusted in the fan shown in FIG. 8. Thisis because the hub 210, or at least that part 270 which contacts theblades 200, forms part of the surface of a sphere centered around apoint 280. The edge 290 of each blade 200 which mates against the hub210 is arranged to fit against the spherical surface of the hub 200. Inthis example, it is a segment of a circle centered on the point 280. Theinner surface of the shroud 220, or at least the part 300 which contactsthe blades 200, forms part of a sphere centered around the point 280.Finally, the outer edge 310 of each blade is again arranged to fitagainst the spherical surface of the shroud 220, and in this exampleforms a segment of a circle centered around the point 280.

In fact, at least a part of each of the hub 210 and the shroud 220 inthis embodiment is frusto-spherical in shape.

Each blade is attached to the hub 210 and to the shroud 220 by pivotableattachment points 320, such as nut and bolt connections. The pivotableattachment points 320 are arranged so that for each blade, the twopivotable attachment points 320 (one on each end of the blade) lie on asingle axis 330 centered on the point 280.

In order to explain how this arrangement allows the blades to bepositioned at different blade angles, it is first noted that a circulardisc of radius r can be positioned at any orientation within a sphere ofinside radius r. Whatever the orientation of the disc within the sphere,however, the center of the disc will lie at the inside surface of theshroud 220 could be considered as part of the inside surface of thesphere referred to above. This means that the outer edge 310 of theblade 200 can be placed at any angle to the inside surface of the shroud220, so long as the center of the curvature of the shroud 220 and theouter edge of the blade 200 remains at the common point 280.Accordingly, the blades 200 can be pivoted around the pivotableattachment points 320 at various angles, but the outer edge 310 of theblade 200 will remain in contact with the inner surface of the shroud220.

This argument can easily be extended to show that the blade angle can bevaried while the inner edge of each blade 200 remains in contact withthe outer surface of the rotating hub 210.

FIG. 9 is a schematic sectional side view of a fan blade 200 for the fanof FIG. 8.

Although the pivot points 320 about which each blade is pivotable forblade angle adjustment should lie on an axis 330 from the common centralpoint 280, it is not in fact necessary for the pivot points to coincidewith the part-circular edges of the blade 200. In fact, the blade 200could pivot around displaced pivot points 340, (e.g., connected to theblades 200 by mounting plates 350). This allows easier access to the nutand bolt connection of the pivotable mounting.

The blade of FIG. 9 is shown having a flat cross-section, but it will beappreciated that the blade could be twisted to give an aero-dynamicshape using known design techniques.

The embodiment of FIG. 8 shows air which is driven by the fan emergingat the motor end of the fan. Similarly, the motor 230 need not bedirectly attached to the hub 210, but could drive via a belt or geararrangement. Various different numbers of blades could be used,depending on the application of the fan.

The alternate embodiments of the mixed flow fan according to theinvention, the performance, characteristics of a mixed flow fan can beobtained, while allowing the performance to be varied easily by changingthe blade angle of incidence. Because the hub and/or shroud surfaces arebased on segments or sections of spherical surfaces, a blade having acomplementary shape at each end can be fixed at different angles betweentwo surfaces.

For ease of adjustment of the fan characteristics, it is preferred thateach blade is pivotable about their respective mounting points. Inparticular, it is also preferred that each blade is pivotable about arespective mounting point on the hub and on the shroud, the mountingpoints on the shroud and the hub lying substantially on the axis ofcurvature of the hub and the shroud.

Other possible modifications include the possibility that the bladesneed not be pivotally mounted with respect to the hub or the shroud. Infact, the blades could be fixed in place, e.g., by welding or brazing,at the time of manufacture. The advantage still remains, however, thatthe fan manufacturer can stock a single pattern of blade and use it toproduce fans featuring a variety of blade angles.

While the fluid impeller of the present invention has been described byreference to its preferred and alternate embodiments, those of ordinaryskill in the art will understand that still other embodiments arepossible based on the embodiments described herein. Such otherembodiments shall fall within the scope of the appended claims.

What is claimed is:
 1. A mixed flow fan comprising: one or more fluidimpelling blades; a hub adjacent to an edge of each of said one or morefluid impelling blades, at least one blade facing surface of said hubbeing formed substantially as a segment of a sphere of radius r₁ about acenter of curvature; a shroud adjacent to an outer edge of each of saidone or more fluid impelling blades, at least one blade facing surface ofthe shroud being formed substantially as a segment of a sphere of radiusr₂ about the center of the curvature; wherein each of said one or morefluid impelling blades has a hub abutting edge curved substantially tofit against a sphere of radius r₁ and a shroud abutting edge curvedsubstantially to fit against a sphere of radius r₂ so that each of saidone or more fluid impelling blades is attachable to said hub and saidshroud at various angles about an axis passing through the center ofcurvature, each angle of said various angles in use achieving adifferent performance and at each angle of said various angles said hubabutting and said shroud abutting edges of one or more fluid impellingblades remain substantially abutting said hub and said shroudrespectively, said hub, said shroud and said blades forming a singlerotatable assembly, each of said one or more fluid impelling blades ispivotable about a respective mounting point an said shroud, each of saidone or more fluid impelling blades is pivotable about a respectivemounting point on said hub, and the mounting points on said shroud andsaid hub lie substantially on an axis passing through the center ofcurvature of said hub and said shroud.
 2. A mixed flow fan according toclaim 1, in which at least part of said hub is frusto-spherical inshape.
 3. A mixed flow fan according to claim 1, in which at least apart of said shroud is frusto-spherical in shape.
 4. A mixed flow fanaccording to claim 1, further comprising drive means for rotatablydriving said one or more fluid impelling blades.
 5. A mixed flow fanaccording to claim 4, in which said drive means is a motor.
 6. A mixedflow fan according to claim 1 in which said hub abutting edge of each ofsaid one or more fluid impelling blades is curved about the center ofcurvature with a radius substantially equal to r₁.
 7. A mixed flow fanaccording to claim 6, in which at least part of said hub isfrusto-spherical in shape.
 8. A mixed flow fan according to claim 6, inwhich at least a part of said shroud is frusto-spherical in shape.
 9. Amixed flow fan according to claim 6, further comprising drive means forrotatably driving said one or more fluid impelling blades.
 10. A mixedflow fan according to claim 9, in which said drive means is a motor.